1. Field of the Invention
The present invention relates to a semiconductor substrate having an SOI (silicon on insulator) structure, and a method for manufacturing a semiconductor device.
2. Description of the Related Art
A semiconductor substrate having an SOI structure that has a thin single crystal semiconductor layer on an insulating layer (hereinafter referred to as an SOI substrate) has been developed instead of a silicon wafer that is manufactured by thinly slicing an ingot of a single crystal semiconductor. Since parasitic capacitance of a transistor is small in an integrated circuit provided over an SOI substrate, it is considered to be effective for improvement of operation speed and reduction in consumption power. Thus, application of an SOI substrate to high-performance semiconductor devices such as a microprocessor has been expected.
As a method for manufacturing an SOI substrate, a hydrogen ion implantation separation method is known (e.g., see Reference 1: U.S. Pat. No. 6,372,609). The hydrogen ion implantation separation method is a technique by which hydrogen ions are implanted into a silicon wafer to form a microbubble layer at a predetermined depth from a surface, and the silicon wafer is bonded to another silicon wafer using the microbubble layer as a cleavage plane so that a thin silicon layer (hereinafter referred to as an SOI layer) is formed over the another silicon wafer. According to this technique, in addition to heat treatment for separation of an SOI layer, it is necessary to perform heat treatment in an oxidizing atmosphere to form an oxide film on the SOI layer, remove the oxide film, and perform heat treatment at 1000° C. to 1300° C. in a reducing atmosphere to increase bonding strength. To increase the bonding strength, to enhance flatness of a surface to be bonded is considered to be effective.
On the other hand, a semiconductor device, in which a single crystal silicon layer is provided over an insulating substrate such as a high heat-resistant glass substrate, is disclosed (see Reference 2: Japanese Published Patent Application No. H11-163363). The semiconductor device has a structure in which the entire surface of crystallized glass having a strain point of 750° C. or higher is protected with an insulating silicon film, and a single crystal silicon layer obtained by a hydrogen ion implantation separation method is bonded firmly to the insulating silicon film.
Now, it is known that a flexible semiconductor device can be manufactured by using a flexible substrate as a substrate provided with a semiconductor device (see reference 3: Japanese Published Patent Application No. 2003-204049). As a method for manufacturing a semiconductor device having a semiconductor layer over a flexible substrate, there is a method in which a semiconductor layer is directly formed over a flexible substrate. However, a method for forming a separation layer over a substrate which has a certain heat resistance; forming an element which has a semiconductor layer over the separation layer; and transferring the element to the flexible substrate using the separation layer as a trigger, is preferably used. At this time, the separation layer is provided, for example, by a sputtering method or the like.